1. Field of the Invention
The present invention generally relates to black level compensation (BLC) for an image sensor, and more particularly to an analog BLC circuit for an image sensor.
2. Description of the Prior Art
Semiconductor based image sensors such as charge-coupled devices (CCDs) or complementary metal-oxide-semiconductor (CMOS) sensors are widely used, for example, in cameras and camcorders, to convert images of visible light into electronic signals that can be stored, transmitted or displayed.
Due to imperfections in electronic circuitry, leakage current exists even when no light is received by the image sensor. In order to overcome this problem, signals (optical black signals) from rows of optically black (or light-shielded) pixels are read from a programmable gain amplifier (PGA) and then averaged as a base reference for the black color to facilitate compensation of the optical black signals. This base reference, however, may vary due to different gain settings in the PGA.
FIG. 1 shows a conventional black level compensation (BLC) system, which includes a digital loop made of an analog-to-digital converter (ADC) 1, a digital circuit 2 and a digital-to-analog converter (DAC) 3. Specifically, compensation for optical black signals of the image sensor begins with the ADC 1 receiving optical black signals from the PGA 4, and accordingly outputting their digital equivalents. The digitized optical black signals are then compared and averaged by the digital circuit 2 to generate an averaged signal. The averaged signal is then converted back to its analog equivalent, which is then used to compensate the PGA 4. The ADC 1 and the digital circuit 2 incur latency, and the DAC 3 is subject to quantization errors, thus attenuating the speed and accuracy of the conventional BLC system.
For the reason that a conventional BLC system, particularly a digital-loop BLC such as described above, cannot speedily and accurately compensate the black level for the image sensor, a need has arisen to propose a novel scheme for rapidly and accurately performing BLC.